The present invention relates to a drilling tool. The drilling tool consists of a drill shank having a drill point and a transition section which adjoins the drill shank in the axial direction which merges into a clamping shank.
Such drilling tools have been disclosed, for example, by German Utility Model G 92 16 228. Disclosed in this document is a twist drill and tap which additionally has a countersinking section in front of the clamping shank.
A tool which is disclosed in EP 0 675 782 B1 and which is not of the generic type, is a torque-transmitting tool which is preferably designed as a screwdriver or screwdriver insert. In this case, the tool consists of a hardened steel body which is produced from a uniform material and whose shank region has subsequently been changed in its hardness or strength by the application of heat. However, the driven region and the driving region have not been changed in their hardness or strength. Such torque-transmitting tools are subjected to torsional stress. Annealing is intended to achieve the effect that the shank region twists elastically during torque application until a limit torque is reached, and can be plastically deformed by at least 30° if the limit torque is exceeded.
In a drilling tool of the generic type, it has been found that, when drilling with a portable drill for example, moment reactions occur when the direction of rotation is reversed. For example, after the countersinking these moment reactions cause the user to experience such a counter-reaction that the drilling tool may fracture.
The object of the invention is to provide an improved drilling tool in which the risk of fracture is considerably reduced.
According to the invention, this is achieved in that the transition section is heat-treated in such a way that it has higher bending and torsional elasticity compared with the clamping shank and/or the drill shank.
When the direction of rotation is reversed, the drilling tool can be jerked suddenly. The invention is based on the knowledge that the drilling tool is stressed in particular, in bending and torsion as a result. The known drilling tool fractures due to this torsional stress. To be precise, the fracture occurs in the vicinity of the tap section in a region which amounts to about 30% of the length of the transition section, starting from the end of the tap section.
The bending and torsional elasticity of the drilling tool is improved by the heat treatment according to the invention so that the drilling tool can largely absorb the bending stresses which occur.
The heat-treated drilling tool heat-treated according to the invention is preferably a combined twist drill and tap having a drill shank which consists of a front twist-drill section and a tap section which adjoins the latter, and merges into the transition section. The clamping shank adjoins the transition section in the axial direction with the countersinking section being arranged between the transition section and the clamping shank.
The heat treatment according to the invention achieves the effect that the transition section is substantially more elastic in bending and torsion than the drill shank and/or the clamping shank with the countersinking section located in front.
According to the invention, provision is made for the drilling tool to have a heat-affected region with a main heat zone. The main heat zone lies approximately in a region which amounts to ⅓ of the length of the transition section, starting from the end of the tap section so that the region subjected to the greatest loading is at least heat-treated. The heat-affected region extends on that side of the transition section which points towards the tap section somewhat into the tap section, and ends on the side pointing towards the clamping shank well in front of the countersinking section. The heat-affected region may end, for example, at about half the length of the transition section in front of the countersinking region. However, the heat-affected region may also end directly in front of the countersinking region. This ensures that the countersinking section has the same material properties as the clamping shank and the drill point.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.